The present invention relates to single screw compressors and more particularly to a scavenging system for same.
In order to form single screw compressors, or positive displacement type machines for varying the pressure of a fluid such as air or gas, it is known to make use of combinations comprising a mainrotor, having a toroidal surface and projecting threads having a generally helicoidal shape e.g. U.S. Patent to Zimmern U.S. Pat. No. Re. 30,400. The crests of the threads are intended to cooperate with a casing, thereby forming compression chambers, and the mainrotor is adapted to cooperate with one or a number of gaterotors, the teeth of which are in meshing relation with the threads formed on the mainrotor.
The space formed between two adjacent threads of a mainrotor of this type can accordingly form a compression chamber which is sealed off at one end by a tooth of one of the gaterotors and sealed off at the other end by providing the casing with a closed end.
When a fluid such as air or gas, which can be at atmospheric pressure, is sucked into a compression chamber of this type, the rotation of the mainrotor permits a progressive reduction in the volume of the compression chamber, compressing the fluid until the compression chamber is put into communication with an outlet which can be formed in the casing.
Because there is relative motion between the parts in the single screw compressor, the clearance between the parts can only be reduced to a minimum finite valve. Even when clearances have been reduced to operational minimums, there are still a large number of paths where the fluid being compressed can leak out.
Current practice in the design and manufacture of single screw compressors, to stem compressed fluid leaks, is to provide liquid seals e.g. U.S. Patent to Zimmern U.S. Pat. No. 3,133,695. By reason of the great speed with which heat is exchanged between the liquids seals and the metallic surfaces in contact with the fluid being compressed, the liquid seals also serve to cool the metallic surfaces being heated by that compressing fluid.
These liquids seals provide an almost perfect sealing since near the leakage zones the presence of a stream of liquid suffices to prevent the escape of gas, because the liquid has a much higher viscosity and inertia.
Typically a liquid seal such as water is injected in the intake area of the mainrotor. This liquid seal, driven along the threads of the mainrotor passes through the clearances between the mainrotor and the gaterotors, and is projected by centrifugal force against the walls of the casing where it forms a seal between the crests of the threads and the walls of the casing.
In practice, at the end of several cycles of operation all the threads of the mainrotor are covered by a film of water which forms an effective seal between the various compression chambers, liquid seal accumulating at the base of each thread until it can pass out through the fluid outlet for the compressed gases. It should be noted that the streams of liquid seal which flow along the threads of the mainrotor are subjected to the pressure of the compressed gas, which tends to force this liquid seal through the clearances between the casing and the mainrotor toward the regions in which the gas is at a lower pressure, that is to say, toward the upper compression chambers, the liquid seal which thus leaks through being recovered in the next compression chamber.
However, the only place where sealing is needed of course is where gas is under compression. Typically a single screw compressor with two gaterotors has a number of compression chambers which are not compressing gas at a particular moment. For a gas in a compression chamber to be under compression it must be sealed by the casing at its crest, at one end by a casing closed end and at the other end by a gaterotor tooth.
Liquid seal injection into the compression chambers in a controlled manner effects proper sealing. However as noted supra some liquid seal is leaked from the compression chambers under compression to the upper chambers which are still not closed by a gaterotor tooth. And as noted supra this residual liquid seal is recycled back into the system. But this recycling is in an uncontrolled manner in the sense that the predetermined liquid seal injected in a controlled manner is supplemented each cycle. The ultimate result is that in effect an excessive amount of liquid seal is injected into the compression chamber, which displaces gas that is under compression, adversely affecting compressor design criteria.